The invention relates to an apparatus for the photo-initiated chemical cross-linking of material that is enclosed in an optically transparent mould for the formation of one or more mouldings, which apparatus has at least one light source by which the material can be acted upon by a light that triggers the cross-linking.
In the chemical cross-linking of material, the molecular structure of the material is altered by the joining of chains of molecules. The material may undergo macroscopic changes during the process, for example it may change from one state of aggregation to a different state of aggregation, for example from the liquid state to the solid state. The cross-linking can be effected, for example, by irradiation with electromagnetic waves (here generally referred to as light). In order to trigger the process, the photons must have a certain minimum energy, with the result that, typically, light in the ultraviolet (UV) wave range is used.
When such a liquid material is enclosed in an optically transparent moulding tool, a chemical process that causes cross-linking in the material can be triggered by exposure to light. The material then becomes solid and retains the shape of the moulding tool. The material moulded in that manner is then removed from the moulding tool, solid mouldings of the desired shape being obtained.
When a mould is used in which a plurality of mouldings are to be produced, it is expedient for those mouldings to be joined to one another in the mould. The liquid material that is to be cross-linked can then be introduced at one site on the mould and is then able to distribute itself and fill up the entire mould. Problems occur, however, when the individual mouldings are to be separated from one another after cross-linking. The parting sites often need to be processed thereafter, for example polished. The same problem occurs also in the case of only one moulding since the site at which the material to be cross-linked is introduced often leaves traces on the moulding itself.
The problem underlying the invention is so to form the beam path in an apparatus of the type mentioned in the introduction that the light that triggers the cross-linking assists in the formation of the outline of the moulding(s).
The problem underlying the invention is especially so to form the beam path in an apparatus of the type mentioned in the introduction that the surfaces of the moulding(s) that are parallel to the optical axis of the apparatus are determined by the beam path of the light that triggers the cross-linking.
According to the invention, that problem is solved in that the region(s) to be cross-linked in the mould is/are determined at least partially by beam-delimiting elements between the light source and the mould. As a result, the region to be cross-linked can be actively controlled.
Such a beam-delimiting element may be, for example, a mask having transparent and non-transparent surface portions which is arranged between the light source and the mould, the mask being projected onto the material that is enclosed in the mould by projection optics.
A specific desired pattern is defined by the mask. That pattern then determines the radial shape of the moulding(s), relative to the optical axis of the apparatus. Even if the size of the mould is greater in that radial direction, the material outside the pattern is not cross-linked and can simply be rinsed off the moulding(s) after the moulding(s) has/have been removed.
Preferably, the light source should deliver as uniform as possible a flow of energy through the desired volume of the material to be cross-linked and the total energy or intensity should be high enough for the chemical process to be completed within as short a time as possible, enabling a good economically viable yield within a production process.
The light source may be optically approximately in point form and condenser optics may be so arranged between the light source and the mask that a largely homogeneous illumination of the mask is obtained. That measure is known and is not discussed in greater detail here.
The apparatus may be so constructed that the projection of the mask onto the material is effected in a telecentric beam path. The telecentric beam path may be either on the object-side side or on the image-side. An object-side telecentric beam path is defined as a beam path where the entrance pupil of the optical system lies at infinity, that is to say the exit pupil coincides with the image focal point. An image-side telecentric beam path is defined as a beam path where the exit pupil of the optical system lies at infinity, that is to say the entrance pupil coincides with the object focal point. When the telecentric beam path is on the image-side, the size of the image does not depend upon the position of the image. In the present invention that is highly advantageous since the mouldings to be formed have a certain extent in the direction of the optical axis of the system. Side surfaces of the mouldings that extend parallel to the optical axis do not, as a result, suffer any curvature during cross-linking. Out-of-focus effects as a result of varied distance and material thickness are minimised.
The light source may be a pulsed UV light source (flashlamp). The cw high performance halogen lamps (cw=continuous wave) that are typically used as light source, which take up a substantial amount of space, have an electrical output in the range of a few kW and are expensive, have a high rate of wear-and-tear, require complicated electrical control and have a low UV yield. A pulsed light source has a very high UV component combined with a distinctly lower average electrical output. That stems from the fact that the UV component is a function of the plasma temperature of the arc and the total electrical output is restricted by the material of the lamps (average thermal load limit). In pulsed lamps the incandescent plasma is substantially hotter, that is to say there are distinctly higher outputs within the individual light pulse. In such lamps, the thermal shock resistance of the lamp is the only limiting factor for the output in the individual pulse. The average electrical output is, on the other hand, low (for example a few watts). Such lamps are also cheaper. Typically, pulsed lamps achieve 200 to 300 times the UV component of cw-operated lamps.
A further advantage of pulsed lamps is that, owing to the lower electrical output, their construction volume is smaller. As a result, they can easily be integrated into an optical system and can be arranged closely together with the optical system in groups.
The pulsed-mode operation also enables exact dimensioning of the amount of irradiation in multiples of the individual pulses by simply counting the pulses per projector.
A retroreflector may be arranged behind the mould, as viewed from the light source The integration of a retroreflector into the beam path after the active volume has been irradiated causes the light to be projected through the desired volume again. Since, typically, only a small proportion is absorbed (that is to say, triggers the desired chemical reaction) in a single passage of the beam, the degree of efficiency of the overall arrangement is almost doubled. Moreover, the projection back into the projector lamp massively increases the plasma temperature of the discharge, which results in a desired intensification of the UV component in the emission spectrum.
For the selection of a suitable energy range, either optical filters may be integrated into the mask, or the collimator, projection optics or moulding tool may be appropriately selectively transparent.
As a result of the present invention, inter alia the exposure to light is homogeneous over relatively large surface areas and, moreover, can be altered individually in fixed portions of the total surface area, which is especially important in the case of multiple processing (that is to say when a plurality of mouldings lie adjacent to one another in a mould) and when small mouldings are being produced.